# Timing Circuit

Discussion in 'Electronic Repair' started by [email protected], Jan 29, 2006.

1. ### Guest

I need to design a circuit that I can tell it to start (digitally), and
then it will tell me when 10 minutes have passed. I just need to be
pointed in the right direction. What's the difference bewteen a
timer/clock/counter. I have a good understanding of circuit theory but
does anyone know of a good IC that can be surface mounted (and is
cheap) that I shoulod use. Any help or tips are greatly appreciated

2. ### Arfa DailyGuest

All depends on how accurate that you need to be really. A simple timer
circuit - the CMOS version of the good old 555 is quite capable of this sort
of delay - could be set up fairly precisely in monostable mode, to do 10
minutes or thereabouts, say give or take a few seconds.

If you really need an accurate 10 minutes, then you need to use a counter.
You would start with a clock generator, probably a standard watch crystal at
32.768kHz, then divide it down to get your 10 minute interval. If you don't
want to divide as far, you can start with a lower frequency, but it's
usually convenient to use a ' binary ' frequency. A 16 bit counter on the
end of a 32k clock, will get you down to a second. You then need a further
10 bits of counter and a bit of AND-ing to get you to the 10 minutes.

You could also pick a ' custom ' frequency for the clock to suit your final
time, which saves the gates at the end.

Horses for courses really

Arfa

3. ### Mr FixitGuest

http://www.maplin.co.uk/Module.aspx?ModuleNo=36130&doy=search

4. ### David NebenzahlGuest

Arfa Daily spake thus:
Since it looks like 32.768 kHz is the lowest-frequency oscillator
available (checked both Digikey & Jameco), I'd say all you need would be
one of them (a complete oscillator, not a crystal) and 2 counter chips
(like the 74HCT4040), one set to divide/16 as you explained, the other
/10. This would give you a TTL pulse at the end of the countdown cycle.

5. ### Fred McKenzieGuest

James-

A simple timer might consist of a resistor charging a capacitor, with a
level detector used to determine when the desired time has been reached.
A more sophisticated timer would use flip-flops to count pulses, with
logic gates set to output a pulse when the state of the flip-flops reaches
a predetermined value.

A "counter" usually refers to an arrangement where counting is done only
during a precise period called the timebase. The result is counts per
unit time, usually expressed in Hertz (Hz), or counts per second. A
free-running counter would continue counting until told to stop, and the
results would just be the number of pulses received since the count was
started.

There is an IC that might get you started, the NE-555. Other sources make
it, but always with the "555" number.

This chip can be configured to start timing on command, and charge-up a
capacitor through a resistor, until a preset time has past. At that point
it should reset and generate an output pulse or level shift.

The trouble with this approach is that it is best for short time delays.
Ten minutes is a relatively long time, so circuit leakage may cause the
delay to be longer than what you would calculate based on the R-C time
constant.

Another approach to timing, is to count a clock, such as the NE-555 being
used as an oscillator or using the 60 Hz powerline frequency. The counter
would be started by your digital signal. When the counter reaches a
predetermined value, it should reset itself and generate an output pulse
or level shift. This method is better for long delays, and is less
affected by circuit tolerances.

Does this help?

Fred

6. ### Dave Plowman (News)Guest

If you use the CMOS version - usually 7555 - the low current allows times
of much more than 10 minutes.

7. ### Laurence TaylorGuest

The problem is that you end up needing absurd component values - ten
minutes delay would need 1.2Mohms and 470mf, for example, and the
latter is likely to be inaccurate and probably variable.

You can use two of them (or a 556), or there used to be a chip that
would work up to many hours delay, the number of which I cannot
recall; I think it was a TDA- something.

--

rgds
LAurence

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8. ### David NebenzahlGuest

Laurence Taylor spake thus:
Why would you want to mess with a 555 at all? Not precise, and more
complicated. I already posted a possible solution using an oscillator
(1MHz) and two 74HCT4040 chips, which will give you an *exact* time and
uses fewer components. (Like no resistors or capacitors, except for
maybe one 0.01 uf bypass cap.)

--
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and interpret information, did not see Hamas doing very well in the
Palestinian election in the wake of these other Islamist victories,
then it is either willfully blind or totally incompetent—-
and neither possibility is a very comforting thought.

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9. ### Guest

Well letcher laurence....
Does everyone know that you are a pedophile?
laurence likes to **** children.....
alt.support.boy-lovers